I've seen the reason come up a few times during my research into it.
I believe this could be underdiagnosed though as I've found that when people ARE tested for D-Lactic Acidosis, they had to struggle with their Doctor before hand to get the test.
But why isn´t it treated when the test is positive for d-lactate hyperlactemia? Seems weird if the doctor who ordered the test does´t treat it...